The high definition television (HDTV) standard for U.S. terrestrial television broadcasts, known as 8 vestigial sideband (8-VSB) modulation was adopted in 1995 by the Advanced Television System Committee (ATSC). The standard (known as the “8-VSB ATSC standard”) specifies single carrier modulation designed for broadcast of high quality video, audio and ancillary data, which supports a payload up to 19.39 Mbps data over a 6 MHz bandwidth channel. Encoded compressed video and AC-3 audio sub-streams are multiplexed with data and service information in packets in an MPEG2 packet stream. The packets are multiplexed and broadcast into the UHF/VHF television spectrum band with an 8-VSB modulator.
In the 8-VSB ATSC standard forward error correcting (FEC) coding techniques are employed to protect the transmitted data against noise. Transmitted data is first coded using a Reed Solomon (R/S) coder and then further coded using a trellis coder Details are given in A53-Annex C. The R/S encoder uses a R/S block code that codes 187 byte blocks into 207 byte blocks, allowing up to 10 bytes of error correction. Each byte of data is segmented into four groups of 2-bit nibbles (x1, x2) prior to being coded with the trellis coder. More precisely, each 2-bit nibble is mapped (coded) using a ⅔ trellis code into a three bit symbol which is associated to points in the signal set {−7, −5, −3, −1, +1, +3, +5, +7}. Each trellis coded symbol is modulated using an 8-level VSB signal.
As a result, a receiver detects modulated signals using a conventional trellis decoding algorithm (such as, for example, the Viterbi algorithm), reducing the likelihood of errors. Additional remaining errors in the decoded stream may be corrected using the R/S codes in stream.
More recently, an enhanced 8-VSB coding technique (EVSB) has been proposed to add flexibility to the 8-VSB standard. Aspects of the EVSB technique are described in U.S. Patent Publication 2004/0028076, the contents of which are hereby incorporated by reference. Notably, EVSB allows for greater immunity to noise than the 8-VSB ATSC standard by including additional coding. Coded symbols within EVSB that are more resistant to noise are referred to as “robust symbols”. Roughly, EVSB robust symbols divide the signal to noise threshold of visibility by two at the cost of reducing the data rate by about the same factor. At the same time, EVSB is backward compatible with the existing 8-VSB ATSC standard. Additionally, 8-VSB ATSC compliant, legacy receivers that are not able to demodulate EVSB robust symbols, seamlessly discard these symbols without jeopardizing normal symbols reception
Bytes encoded using a robust trellis (hereinafter “robust bytes”) and bytes encoded using conventional VSB coding (hereinafter “normal bytes”) may be interleaved. The interleaving of robust bytes and normal bytes results in interleaved robust/normal symbols formed using two different convolutional codes. As a consequence, an EVSB capable receiver should be able to decode a stream of symbols formed from two different trellis codes. Convolutional and trellis codes are for example detailed in Lin, Shu & d. Costello, Error Control Coding, Prentice-Hall, 1983, the contents of which are hereby incorporated herein by reference.
To this end, the robust convolutional code leading to the generation of the robust symbols (via a trellis code) is chosen so that normal symbols in a normal/robust stream can be decoded by a conventional 8-VSB trellis decoder. At the same time, a conventional trellis decoder similar to the one used for 8-VSB encoding but adapted to the EVSB trellis coder can decode both normal and robust symbols in the stream.
As will be appreciated, trellis codes are convolutional codes that encode sequences of symbols, rather than individual symbols. As such, the performance of a trellis decoder typically depends on the number of symbols used to produce each individual decoded symbol. The number of symbols used is also often referred to as the “window” of received symbols. A minimum length window is required to achieve acceptable performance. Practically, the length of the window is fixed and limited by hardware cost. In an EVSB stream, the number of robust symbols and normal symbols received vary in dependence on the mix of normal and robust symbols sent by the transmitter, as controlled by the broadcaster. Because normal symbols are less immune to noise than robust symbols, the ability to estimate the robust symbols depends on how many robust symbols are in the window. This will typically be affected by the number of normal symbols within the window. In particular, to achieve adequate estimations of robust symbols at a low robust to normal symbol ratio, the length of window needs to be large, and is often impractical.
Accordingly, there is a need for an improved receiver that allows for optimum performance for the estimate of robust symbols with a fixed window length used to decode streams including robust and normal symbols.